Definition of Geological Structures

• Geological structures refer to the arrangement and deformation of rocks in the Earth’s crust due to tectonic forces, stress, and strain.
• They provide evidence of past geological events such as mountain formation, earthquakes, and plate movements.
• Significance:
  • Helps in geological mapping and resource exploration.
    
  • Provides insights into earthquake risks and tectonic activity.

Forces Acting on Rocks

Stress vs. Strain

• Stress → The force applied to a rock.
• Strain → The deformation caused by stress.

Types of Tectonic Forces:

1. Tension → Stretches rocks apart.
2. Compression → Pushes rocks together.
3. Torsion → Twisting forces.
4. Shear → Sideways motion.

Types of Deformation

1. Elastic Deformation → Temporary, rocks return to original shape.
2. Plastic (Ductile) Deformation → Permanent bending.
3. Rupture (Brittle) Deformation → Rocks break (faulting).

Classification of Geological structures

• Primary Structures → Formed during rock deposition.
  
• Secondary Structures → Formed due to later deformation.
  
• Unconformities → Gaps in geological records.

Primary Structures

• Definition:
  • Structures formed during sediment deposition or rock formation.
• Examples:
  • Stratification (Bedding)
  • Graded Bedding
  • Cross Bedding
  • Ripple Marks
  • Mud Cracks
• These were discussed in the previous lecture on sedimentary structures.

Secondary Structures

• Definition:
  • Structures formed due to tectonic forces, stress, and deformation.
• Includes:
  1. Folds
  2. Faults
  3. Joints

Folds

Definition:

• Bending of rock layers due to compression forces.

Fold Geometry:

• Axial Plane → Divides the fold into two limbs.
• Crest → Highest point of a fold.
• Trough → Lowest point of a fold.
• Limb → Sides of the fold.

Types of Folds

Based on Symmetry:

• Symmetrical Folds → Limbs are equally inclined.
• Asymmetrical Folds → Limbs dip at different angles.

Based on Limb Inclination:

• Monocline Fold → One limb is inclined.
• Anticline → Convex-upward fold (oldest rocks in the center).
• Syncline → Concave-downward fold (youngest rocks in the center).
• Overturned Fold → One limb is tilted beyond vertical.
• Recumbent Fold → Horizontal fold.

Based on Fold Axis:

• Non-plunging Fold → Horizontal fold axis.
• Plunging Fold → Fold axis dips into the ground.

Other Fold Structures

• Domal Structure → Upward-arching structure.
• Basin → Downward-sinking structure.

Faults

Definition:

• A fracture in Earth’s crust along which movement has occurred due to stress.

Fault Geometry:

• Fault Plane → Surface along which movement occurs.
• Strike → Direction of the fault plane.
• Dip → Angle of inclination of the fault.
• Downthrown Side → Side that moved down.
• Upthrown Side → Side that moved up.
• Hanging Wall → Above the fault plane.
• Foot Wall → Below the fault plane.

Classification of Faults

1. Dip-Slip Faults:
   • Normal Fault → Hanging wall moves down (due to tension).
   • Reverse Fault → Hanging wall moves up (due to compression).

Classification of Faults

2. Strike-Slip Faults:
   • Left-Lateral → Opposite side moves left.
   • Right-Lateral → Opposite side moves right.

Classification of Faults

3. Other Faults:
   • Thrust Fault → Low-angle reverse fault.
   • Transform Fault → Large-scale strike-slip fault (e.g., San Andreas Fault).
   • Oblique Fault → Combination of dip-slip and strike-slip motion.
   • Graben Fault → Down-dropped block between two normal faults.

Evidence of Faults

• Displacement → Visible offset of rock layers.
• Fault Breccia → Broken rock fragments along the fault.
• Slickensides → Polished fault surfaces due to friction.
• Drag Folds → Folding along a fault line.
• Silicification & Mineralization → Mineral deposits along faults.
• Topographic Changes → Fault scarps, cliffs, and valleys.

Joints

Definition:

• Fractures in rocks without displacement.

Formation:

• Due to cooling, unloading, or tectonic stress.

Types of Joints:

• Vertical Joints → Formed by extension forces.
• Horizontal Joints → Due to pressure release (exfoliation).
• Oblique Joints → Formed by combined forces.
• Columnar Joints → Hexagonal columns in cooling lava (e.g., Giant’s Causeway).

Unconformity

Definition:

• A break in the geological record due to erosion or non-deposition.

Types of Unconformities:

1. Angular Unconformity → Tilted or folded rocks overlain by flat layers.
2. Nonconformity → Igneous/metamorphic rocks overlain by sedimentary rocks.
3. Disconformity → Parallel sedimentary layers with an erosional gap.

Evidence of Unconformity:

• Erosional Surfaces
• Fossil Gaps
• Changes in Rock Type